It is known to equip engines with exhaust gas recirculation (EGR) systems. Such systems generally comprise a branch conduit between the inlet manifold and the exhaust pipe of an engine. A valve is placed in the branch conduit and is adjustable to recirculate a fraction of the exhaust gases generated by the engine. Thus, a smaller portion of the exhaust gases is taken from the exhaust and mixed with the intake gases. In order to increase the density of the intake charge, the recirculated gases are normally passed through an intercooler in the branch conduit, thereby lowering the temperature of the intake charge.
The above-described systems are primarily used to reduce harmful exhaust emissions, i.e. nitrous oxides (NO.sub.x), mainly for automobile engines. Due to the introduction of cooled recirculated exhaust gas, the peak temperature in the combustion chamber is lowered during the combustion phase, thus limiting the formation of NO.sub.x. Normally, up to between about 5 percent and 20 percent of the exhaust gas volume is recycled. The EGR system is traditionally shut off when the engine load is low, e.g. during idling, to lower the fuel consumption and, in the case of petrol engines, to achieve a more even running of the engine. This also serves to prevent fouling of the valves and EGR system.
It is also known to utilise exhaust brakes in diesel engined vehicles, see U.S. Pat. No. 5,193,657. Such exhaust brakes are activated when the engine speed has to be lowered rapidly or when the engine-braking efficiency has to be increased, for instance when the driver of the vehicle presses the brake pedal. More efficient engine braking is achieved because the engine load is increased.
Increased exhaust pressure, and thereby increased load on the engine, is often utilized by truck drivers when their vehicles are stationary for relatively long periods in cold climates. Due to the increased load on the engine, more fuel is burnt and therefore the engine temperature is increased. The heat thus generated is then used for heating the interior of the vehicle cab. The increased load on the engine does, however, increase the fuel consumption of the engine.
From the documents JP-A-7180572 and WO-A-9310339 it is known to use an exhaust pressure governor (EPG) to adjust the exhaust back pressure to vary the exhaust flow depending upon the operating state of a spark-ignition engine. This is said to result in improved combustion efficiency and a raised catalytic converter temperature, with a resulting decrease in NO.sub.x emissions through the exhaust.
From DE-A-3339053 it is known to briefly completely shut off the exhaust when the brakes are applied, when starting the engine or when the engine is idling. According to said document, this results in lowered emission of unburnt white fumes when cold starting and lowered emission of blue smoke when idling.